1. Field of the Invention
The present invention relates to a liquid delivery system and more particularly, to a liquid delivery system for delivering a liquid source material at a specified rate to an equipment, which is preferably used for semiconductor device fabrication such as chemical vapor deposition (CVD) and dry etching.
2. Description of the Prior Art
Conventionally, when a source material in liquid form at ambient temperature is delivered to a CVD equipment at a specified flow rate, the following two delivering methods have been employed.
A first delivering method is advantageous to liquid source materials having a comparatively high vapor pressure such as titanium tetrachrolide (TiCl.sub.4). In this method, a carrier gas such as an inert gas is injected into the liquid source material stored in a container for bubbling, thereby producing a vapor of the liquid source material contained in the carrier gas. Then, the carrier gas containing the source the equipment through transferring ducts or channels.
Thus, the liquid source material is bubbled ad then, it is transported or delivered to the chamber together with the carrier gas. The first delivering method has been practically used most popularly because of its easiness.
A second delivering method is advantageous to liquid source materials having a low vapor pressure such as tetrakis-diethylamino-titanium Ti[N(C.sub.2 H.sub.5).sub.2 ].sub.4, because these source materials cannot be delivered at a desired flow rate by the bubbling technique. In this method, the source material is metered in liquid form with a mass flow controller, a metering pump or the like and then, it is transported to a vaporizer. The liquid source material thus transported is vaporized in the vaporizer and is delivered to a reaction chamber of the CVD equipment through delivering channels.
Thus, the liquid source material is metered in liquid form without the bubbling process and it is transported to the vaporizer placed on the upstream-side of the CVD chamber. No carrier gas is required.
The mass flow controller used in the second delivering method, which is designed for liquids, has a similar configuration to popular mass flow controllers designed for gasses. Specifically, the controller designed for liquids has a flow rate sensor placed in a capillary of the main delivering channel, a conductance-variable valve which is capable of high-speed response, and an electronic control system for controlling the flow rate of the source material in the main channel. The flow rate of the liquid source material is always monitored by the sensor. The conductance of the valve varies in response to the output signal from the sensor under the feed-back control by the control system, thereby keeping the flow rate of the source material at a specified delivering rate.
FIG. 1 shows a conventional liquid delivery system for delivering a liquid source material at a specified rate to a CVD equipment, which has been usually employed for performing the above second delivering method.
As shown in FIG. 1, this delivery system includes a conductance-variable valve 35 capable of high-speed response, a valve actuator 34 for driving the valve 35, and a flow rate sensor 36 for sensing the flow rate of a liquid source material 32 which is in liquid form at ambient temperature.
The liquid source material 32 is stored in a source container 31. To apply a specified pressure to the source 32A, a pressurizing gas 33 such as an inert gas is supplied to the container 31. A main duct or channel 38, which is made of metal or plastic tubes, is provided so that the container 31 communicates with an inlet of a CVD equipment.
A metering pump (not shown) is provided at the main channel 38 to transfer the material 32 to the chamber.
The conductance-variable valve 35 is placed on the main channel 38 midway between the container 31 and the deposition chamber. The valve 35 forms a variable orifice 35a in the channel 38. The size of the orifice 35a, that is, the conductance of the valve 35, varies under the feedback control by the valve actuator 34.
The flow rate sensor 36 also is placed at the main channel 38 midway between the container 31 and the deposition chamber. A bypass 37 is formed by a capillary at the main channel 38 on the downstream side of the valve 35. The sensor 36 is provided at the bypass 37, and it senses the flow rate of the liquid source material 32 flowing through the bypass 37 to output an electric signal S to the valve actuator 34. In response to the received signal S, the actuator 36 actuates the valve 35 to change its conductance, thereby delivering the source material 32 to the deposition chamber at a specified, constant flow rate.
The conventional liquid delivery system shown in FIG. 1 has the following problems.
First, the conventional system inherently requires a difference between the liquid pressures at the inlet and outlet of the valve 35, since the conventional system employs the conductance-variable valve 35. To produce this pressure difference, the pressuring gas 33 is supplied to the source container 31 for pressuring the source material 32. This is very popular.
However, the pressuring gas 33 tends to be dissolved in the source material 32, and the dissolved gas 33 tends to become bubbles within the main channel 38. As a result, a first problem that a large or serious error tends to be caused by the sensor 36 in sensing the flow rate and that the main channel 38 and/or the bypass 37 is blocked or choked.
Second, since the movable element or part of the valve 35 (for example, a piston) is typically made of metal or plastic, there is a possibility that the element generates dust. The dust has a tendency to induce contamination of the source material 32.
Third, although the liquid source material 32 flows in the form of laminar flow through the bypass 37 made of the capillary, the flowing velocity of the material 32 is approximately zero (0) in the vicinity of the inner wall of the bypass 37. Therefore, contamination tends to deposited or accumulated on the inner wall, which leads to block of the bypass 37 also.
When the metering pump is of the reciprocating type, the following fourth to sixth problems additionally occur.
The fourth problem is that precise flow rate control is difficult, because flow rate pulsate of the source material 32 takes place due to the reciprocating motion.
The fifth problem is that the source material 32 tends to be contaminated by dusts that is generated by friction in reciprocating motion.
The sixth problem is that when the main channel 38 and/or bypass 37 is blocked, a possibility of damage or destruction of the pump occurs due to pressure rise, and that a firing danger of the source material 32 occurs.